The cytology and developmental genetics of oogenesis has been studied in greater detail in Drosophila melanogaster than in any other species. In this fly the complex steps in the development of an egg can be genetically dissected utilizing mutations belonging to the class called female steriles. Over 200 different female sterile mutations are now available for study, and each represents a gene whose product is essential for one or more steps in oogenesis. Our studies are designed t elucidate the detailed ovarian phenotypes produced by certain specific classes of mutants, namely those affecting (1) the formation of ovarioles, (2) the ransformation of oogenia to cystoblasts, (3) the cytokinesis of cystocytes, (4) the migration and secretory behavior of follicle cells, (5) the transcription and processing of rRNa by nurse cells, and (6) the uptake of vitellogenin by oocytes. The techniques used will include (1) transmission electron microscopy of sectioned ovaries and of whole mounts of isolated chrmatin, (2) scanning electron microscopy of intact and dissociated egg chambers, (3) ovary transplantations into hosts of different genotypes, and (4) localization of the mutations under study to specific chromosomal regions using conventional cytogenetic techniques.